Dave, when I started this thread, I didn't realize that Readability-Score.com belongs to you. Nice work! Congratulations!
I am still using HemingwayApp editor. They charge a small amount of money to use the downloadable version - it was $10 or less. I've paid to use it on several computers. I'd pay more, a lot more, if it stored all of my documents in a single location that I could access from multiple computers.
Funnily enough, it sounds like my other project, ApolloPad, might be useful to you. It's a web-based writing tool. A bit like an online scrivener. It's got readability baked in, but a bit behind the tools on RS (I'll be pulling over the upgrades in the new year).
Hemingway is great. The text highlighting I added to RS is entirely because I thought theirs was so cool. I've done my highlighting differently - their tool is all JS, mine's server-based - but the ability to pick out problems quickly makes the site so much more useful. If I ever meet them, I'll owe them a few beers.